(1) Field of the Invention
The invention relates to the protection of telephone lines from overvoltages caused by lightning and induction, for example from nearby heavy-current AC supplies.
In particular it relates to the protection of the electronic circuits which are connected in the telephone exchange to the telephone line.
(2) Description of the Prior Art
Known means for the coarse protection of telephone line include fuses, lightning protectors in the form of a narrow gap between two conducting electrodes (carbon blocks), gas discharge protectors and so-called heat coils.
In order to provide fine protection of the telephone lines, semiconductor diodes and zener diodes are used which clamp the voltages of the wires of the telephone line to a given level during the occurrence of overvoltages due to lightning or induction from a high-current AC supply. Impedances are usually connected into the telephone line to limit the current, or a portion of the telephone line itself is used as an impedance.
If the wires of the telephone line are clamped to the supply voltages of the electronic circuit by means of limiter diodes and with current limiting resistors in the wires the following phenomena are produced.
During an induction surge the initial voltage across the limiter diode is considerably greater than the steady state value owing to the inertia of the modulation of the diode resistance and owing to the self-induction of the supply leads.
The current which flows through the limiter diode to a supply voltage terminal increases the supply voltage owing to the internal impedance of the power supply source.